1. Field of the Invention
The present invention relates to a press molding method for glass and to a manufacturing method for a glass substrate using this method.
The present invention also relates to molded glass, for example, to a glass substrate, in particular, to a glass substrate of which the outer periphery portion is unprocessed and to a glass substrate provided with a center hole, as well as to manufacturing methods for the above.
2. Description of the Related Art
A type of hard disk utilized as an information recording medium of a computer wherein a base layer, a recording layer and a protective layer are sequentially layered on the surface (including the outer periphery edge surface) of a disk-shaped substrate made of glass is known and is utilized by rotation, having the center of this disk-shaped substrate as the center of rotation. The outer periphery portion of such a glass substrate is processed with precision and the center hole of a glass substrate of a hard disk provided with a center hole is created so that the center of the center hole becomes center of the hard disk.
A manufacturing method for a glass substrate of a hard disk is briefly described using the flow chart of FIG. 18. First, glass material is melted (glass melting step), the melted glass is made to flow into a lower mold and press molding is carried out by means of an upper mold (press molding step). In the press molding step, methods such as are shown in FIGS. 16 and 17 are generally adopted. In FIG. 16, simple press molding is carried out on glass material 103 so that it attains a predetermined thickness utilizing an upper mold 101 and a lower mold 102 provided with molding surfaces having planar forms (Japanese unexamined patent publication H11 (1999)-255524). In FIG. 17, press molding is carried out on glass material 108 when an outer diameter regulating frame 107, in a ring form, is intervened between an upper mold 105 and a lower mold 106 provided with molding surfaces having planar forms. In the method shown in FIG. 17, according to a detailed description, the outer periphery edge surface of glass material 108 contacts outer diameter regulating frame 107, in a ring form, at the time of press molding so that the outer diameter of the glass substrate is regulated (Japanese unexamined patent publication H7 (1995)-133121).
The glass material (glass substrate) on which press molding has been carried out is crystallized or annealed and, then, is cooled (crystallization step or annealing step). A hole is created, if desired, in the center portion of the cooled glass substrate through cutting and, after that, the outer periphery edge portion of the glass substrate, at least, is cut so that the dimensions of the outer diameter and the circularity of the glass substrate are pre-adjusted (coring step or outer periphery pre-processing step). The glass substrate, of which the dimensions of the outer diameter, or the like, have been pre-adjusted, undergoes the first lapping process wherein the two surfaces are polished so that the entire form of the glass substrate, that is to say, the parallelism, the flatness and the thickness of the glass substrate, is pre-adjusted (first lapping step). The outer periphery edge surface, at least, and the inner periphery edge surface of the hole in the glass substrate, if desired, of the glass substrate, of which the parallelism, and the like, are pre-adjusted, are polished or rounded so that the dimensions of the outer diameter and the circularity of the glass substrate, the dimensions of the inner diameter of the hole, as well as the concentricity of the glass substrate and the hole, are microscopically adjusted (precision edge surface processing step (inner and outer)). The outer periphery edge surface, at least, and the inner periphery edge surface of the hole, if desired, of the glass substrate, of which the dimensions of the outer diameter, or the like, have been microscopically adjusted, are polished so that the edge surfaces are made to have a mirror surface (edge surface polishing process (inner and outer)). The two surfaces of the glass substrate, of which the edge surfaces have been polished, are again polished so that the entire form of the glass substrate, that is to say, the parallelism, the flatness and the thickness of the glass substrate, is microscopically adjusted (second lapping step). The glass substrate, of which the parallelism, and the like, has been microscopically adjusted, undergoes the polishing step so that the two surfaces are polished and the unevenness of the surfaces is eliminated (polishing step). The glass substrate, which has been polished, is finally washed and inspected so that only those that have passed inspection can be utilized as substrates for hard disks.
According to conventional methods there is a limit to the degree of the mirror surface of the outer periphery edge surface although mirror surfaces can be achieved on the top surface and on the bottom surface of the glass substrate. That is to say, even though the outer periphery portion of the glass substrate is processed as described above, in particular, even though the outer periphery edge surface of the glass substrate is polished in the edge surface polishing step according to the conventional methods, the outer periphery edge surface cannot be sufficiently converted to a mirror surface because this process for the edge surface is complex and because polishing of this edge surface cannot be carried out for sufficiently long period of time due to manufacturing cost considerations. Described in detail, microscopic scratches remain in the outer periphery edge surface of the glass substrate gained according to the conventional methods so that the surface has a surface coarseness of at least approximately 5 nm and a maximum surface coarseness of approximately 250 nm. When microscopic scratches remain in the outer periphery edge surface in such a manner, it is difficult to layer a base layer, a recording layer, a protective layer, and the like, on top of these microscopic scratches. In addition, even in the case that a base layer, a recording layer, a protective layer, and the like, can be layered on top of the microscopic scratches, the amount of alkaline components exuding out of these microscopic scratches significantly increases as time elapses and, therefore, a base layer, a recording layer, a protective layer, and the like, which have been layered onto the surfaces (including the outer periphery edge surface) are corroded from the outer periphery portion of the substrate at a comparatively early stage so that a problem arises, as a consequence, wherein the stored data is destroyed comparatively easily.
In addition, as for a cutting method in the coring step in the case that a glass substrate of a hard disk provided with a center hole is manufactured, inner and outer diameter processing by means of polishing, a method wherein scribing is carried out on at least one surface of a glass substrate and, then, an impact is given to the portion to be cut in the condition wherein this scribed surface is facing upward (Japanese unexamined patent publication H11 (1999)-116261), a method of dividing a glass substrate by cutting shallow lines into the glass substrate by means of a laser beam (for example, Japanese unexamined patent publication 2000-281371), and the like, are generally adopted. According to the conventional methods, however, a hole is created in a glass substrate, of which the two surfaces are flat through their entirety, in the coring step by means of the above described cutting methods and, therefore, there is the problem of increased cost of manufacture because processing time becomes too lengthy, the utilized devices are too expensive and the processes are too complex.
Furthermore, according to the conventional methods, the precision of the press molding step is particularly poor so that it becomes necessary to adjust the entire form of a glass substrate by means of the above described lapping step, or the like, and because the number of steps is great an increase in the cost of manufacture has become a problem. That is to say, according to the method shown in FIG. 16, the thickness of a glass substrate is controlled according to the distance between the upper mold and the lower mold and, therefore, it is difficult to gain a glass substrate having a predetermined thickness. Furthermore, when glass substrates are repeatedly manufactured, one axis, of the center axis of the upper mold of the center axis of the lower mold, easily becomes tilted relative to the other axis so that the parallelism is deteriorated. In addition, according to the method shown in FIG. 17, glass makes contact with the entirety of the upper mold, the lower mold and the outer diameter regulating frame at the time of press molding and, therefore, a slight increase in the amount of glass prevents pressure from being uniformly conveyed to the glass so that the parallelism and the flatness easily become deteriorated. In addition, when molding is repeatedly carried out, a great dispersion in thickness results.